Cable assemblies with shorts or weak insulation can result in nonfunctional or, worse, unsafe cables. Simple continuity testing isn't enough. Thus, makers of commercial cable assemblies and OEMs test their cables for isolation against high voltages.

High-voltage testing, called hipot testing, is governed by UL (United States), CSA (Canada), and IEC (Europe) standards. As a general rule, testing should be 1,000 V higher than double the operating voltage, but that can vary depending on use and applicable standards. Jorel Townsend, an applications engineer at Chroma USA, tells us that hipot test voltages typically range from 500V to 3kV. A hipot test will detect short circuits and opens in wiring, as well as pinholes in insulation.

A hipot tester applies a test voltage and measures any current that might flow. The allowable current depends on the application and the cable length. To run a hipot test on a cable or cable assembly, you apply the test voltage across every pair of pins or wires, then from every pin to a shield or connector case. To pass a test, the measured current must be below a limit, and a cable assembly should show no arcs or flashes when the test voltage is applied.

A Guardian hipot analyzer from Chroma.

Townsend says hipot testing should be performed on every cable assembly that will be available for sale. Cables manufactured for installation into a larger product don't necessarily need hipot testing, so long as the subassembly or final product gets a test.

Test voltages can be DC or AC. Cables that pass only DC can be tested with either DC or AC voltages, but cables that pass AC need AC voltage tests. That's why some hipot testers produce AC voltages only.

The ratio of AC voltage to the equivalent DC voltage is, as you might expect, 1.414. Multiply the AC voltage by 1.414 to find the DC voltage. Conversely, divide the DC voltage by 1.414 (multiply by 0.707) to find the AC voltage.

Because hipot tests usually involve dangerous voltages, a test setup must be safe to operate. Hipot testers have safety features such as overcurrent protection, and they have an interlock signal input. Overcurrent protection comes in the form of a ground-fault interrupt circuit, like the ones in the AC mains outlets in your home. Use the interlock input with mechanical switches (such as handheld push buttons) to disable the high-voltage output when the switch isn't closed. Another safety technique is to use an infrared sensor that opens the interlock input should anything (such as a hand) break the beam.

You can find many application notes on hipot testing. Here are a few links.

One way to test for pinholes or cracks in a cable's insulation is to immrse it in water. Connect your hipot tester high side to the cable's conductor and the low side to an electrode immersed in the water. We have also used that technique to check the encapsulation of photovoltaic panels.

I kike the idea of safety, but exactly how would a hipot test detect holes in the outside insulation of, say, a shielded cable? It seems the cable must be wrapped in foil, or passed through a conductive and compliant 'doughnut' while 'hot' to do this.